Query Processing

When a port receives an IGMP (Internet Group Management Protocol) query, the snooping switch identifies the receiving port as a multicast router-attached port. The switch maintains the list of multicast router-attached ports on a per-VLAN basis. If the port does not receive periodic IGMP queries, the learned entries maintained in the list expire after a configured interval. The snooping switch stores the last received query on a VLAN because the switch uses this to calculate the max response time value during IGMP Leave message processing.

The snooping switch treats Distance Vector Multicast Routing Protocol (DVMRP) probe messages and Protocol Independent Multicast (PIM) versions 1 and 2 hello messages that it receives similar to IGMP queries by adding the interfaces that receive these messages to the list of multicast router-attached ports.